In recent years, various attempts have been made to separately create the microstructural feature and material property of a metallic material according to the application of metallic material. For example, a method has begun to be used in which, when a metallic material is cooled after hot rolling, cooling water is sprayed at a high pressure in large quantities to increase the cooling rate of steel plate, by which the metallographic structure is changed to provide desired tensile strength or ductility. Conventionally, however, in practically using such a manufacturing method, there has been unavailable a method for efficiently checking whether or not the manufactured metallic material has the required microstructural feature and material property.
Conventionally, the mechanical properties such as tensile strength, ductility, and formability of a metallic material have been measured by a destructive test such as a tensile test. However, such a measuring method requires several hours to several days before the test result is obtained, and also has a problem in that 100% test cannot be performed because of destructive test. Therefore, it has been strongly demanded that the microstructural feature and material property of a metallic material be tested in a non-destructive mode.
To cope with the above problems, as one method for testing the microstructural feature and material property of a metallic material in a non-destructive mode, there has been known a method in which ultrasonic waves are transmitted into a metallic material, for example, a metal piece, and the microstructural feature and material property of a metallic material are monitored based on the propagation characteristics of the ultrasonic waves. With this method, various characteristic values of microstructural feature and material property can be monitored by utilizing any of the oscillation modes of ultrasonic waves. For example, from the attenuation characteristics of the high-frequency component of longitudinal wave, the crystal grain size of metal piece can be detected, and further the measurement values of yield stress and tensile stress, which correlate strongly with the crystal grain size, can be obtained. Also, from the propagation velocity of transverse wave, the modulus of elasticity of metal piece can be detected, and further from the anisotropy of the modulus of elasticity, the measurement value of Lankford value (r value), which is one of the characteristic values of microstructural feature and material property representing the formability of metal piece, can be obtained.
In monitoring the microstructural feature and material property of the metal piece (metallic material) by the above-described method, as means for transmitting ultrasonic waves into the metal piece and means for receiving ultrasonic waves having propagated in the metal piece, for example, a method has been widely known in which a piezoelectric element is brought into contact with the metal piece. In this method, however, the piezoelectric element must be brought into close contact with the metal piece via a liquid etc., so that there arise a problem in that this method is unsuitable for on-line measurement especially on a production line and a problem in that this method is unsuitable for the measurement of crystal grain size because the oscillation frequency is low (<1 MHz).
On the other hand, in recent years, a method has been used in which pulse-shaped ultrasonic waves are transmitted into the metal piece by applying pulse laser beams to the surface of metal piece (for example, refer to Patent Document 1). This method has an advantage that the ultrasonic waves can be transmitted into the metal piece, for example, by applying pulse laser beams from a position separate from the metal piece and an advantage that the pulse ultrasonic waves containing a high-frequency component of several tens megahertz or higher can be transmitted into the metal piece, for example, by decreasing the pulse width of laser beams.
Also, in recent years, a method has been used in which the ultrasonic waves having propagated in the metal piece is received by utilizing a laser interferometer. In this method, by applying laser beams to the metal piece separately from the laser beams for transmitting ultrasonic waves, minute ultrasonic wave oscillations appearing on the surface of metal piece are read by causing the reflected light to interfere with the reference light. Therefore, this method has an advantage that the minute ultrasonic wave oscillations appearing on the surface of metal piece can be received, for example, from a position separate from the metal piece and an advantage that ultrasonic wave oscillations of high frequency of several tens megahertz or higher can be received.
Patent Document 1: Japanese Patent Publication No. 61-54179